The present invention relates to a system for controlling the clutch torque of an electromagnetic clutch for an infinitely variable belt-drive transmission.
Regarding the system for controlling the clutch torque during starting of a vehicle, Japanese Patent Laid-open 57-160724 which corresponds to U.S. Pat. No. 4,518,068 discloses a control system wherein a very small amount of clutch torque is supplied and increased stepwise in relation to engine speed during the initial period of the clutch engagement, in order to establish a smooth and rapid engagement of the clutch when starting a vehicle.
The combination of the above-mentioned type of electromagnetic clutch with an infinitely variable belt-drive transmission system has already been proposed. The infinitely variable belt-drive transmission system enables a continuous infinitely variable change speed. However, there remain many problems to solve that may occur when the system is actually mounted on a vehicle, such as static friction torque generated in the pulleys and belt device. Since the transmission ratio during the starting operation of the vehicle is at maximum value and the power transmission torque at the pulleys and belt device is large, the belt is pressed against the pulleys with a large pressure. Accordingly, the static friction torque becomes large. Therefore, most of the engine power transmitted through the engagement of the clutch when starting the vehicle is consumed by the static friction torque, resulting in a slow-moving start of the vehicle, unpleasant vibration occurring when the static friction torque disappears and dynamic friction torque generated, and noise generated by engagement between the teeth of the gears disposed with backlashes.
To cope with the problems, it has been proposed to apply a very small drag torque to the clutch before starting a vehicle. The generation of the drag torque is caused by detecting a very low vehicle speed and the release of an accelerator pedal. On the other hand, in the infinitely variable belt-drive transmission, during deleration, the transmission is automatically downshifted to a high transmission ratio, and finally to a maximum transmission ratio. At a proper state, it is necessary to disengage the clutch before the vehicle is fully stopped in order to avoid stalling of the engine. However, if the drag torque is generated with the clutch disengaged, the vehicle is greatly braked by the engine braking effect because of the high transmission ratio. Consequently, the coasting performance of the vehicle is reduced.